Brown not only signed a bill that calls for 100% of the state’s electricity to come from carbon-free sources by 2045 but also penned an executive order demanding that California completely eliminate net emissions across its economy, including transport and agriculture, by the same year.

The order, which cites “historic droughts, devastating storms, torrential storms, extreme heat … and threats to human health”, requires California pull more greenhouse gases out of the atmosphere than it puts in by 2046, using vegetation and new technologies.

Brown told the Guardian that California was taking the lead globally in confronting an issue that’s an “existential challenge” to the planet and claimed that Trump was the main obstacle to further progress.

“I don’t believe Trump represents the present but he has the power, he has the Republicans hook, line and sinker,” he said. “His unabashed acolytes will follow him over the cliff. Gross ignorance is dangerous. The battle is with Trump – that’s the number one fight. But once he’s out of there, dealing with climate change will still be a fight.”

The Trump administration has sought to dismantle emissions rules for coal-fired power plants and loosen fuel efficiency standards for cars and trucks, targeting California’s ability to set stricter vehicle regulations than the rest of the country.

Brown has set a target of 5m electric cars on California’s roads but said “at the rate we’re going they will be made by Chinese companies and that will principally be the fault of Donald Trump. The American auto industry is on the chopping block at at the moment he is chopping it into oblivion.”

The vision of carbon neutrality and the banishment of fossil fuels set out by Brown has taken aback even some environmentalists in a state that is a major oil and gas producer, as well as the nation’s agricultural heavyweight.

California will look to soak up carbon through expanded forests, at a time when enormous wildfires are a growing blight on the state, or inject carbon underground or in algal blooms – a technological challenge that has yet to be proven at scale.

“Places like Sweden and Costa Rica have had those kind of pledges but California is the fifth largest economy in the world,” said Nigel Purvis, who oversaw environmental diplomacy at the state department under the Clinton and George W Bush administrations. “This is historic. It’s a really major step. Jerry Brown has been one of America’s great climate leaders.”

Now entering the twilight of his second spell as governor, Brown has forged a reputation for environmentalism, bringing in one of the first ever tax incentives for rooftop solar during his first period as the state’s chief executive, which ran from 1975 to 1983.

His second stint as governor, which began in 2011, has seen Brown throw himself at the issue of climate change, overseeing a reduction in greenhouse gas emissions to 1990 levels during a period of strong economic growth by implementing a carbon-trading scheme that recently expanded in scope to help prevent deforestation overseas.

Latterly, his tenure has brought him into diametric opposition to Trump. Brown has compared the president’s climate policies to falling off the top of the Empire State Building and said California will “launch its own damn satellite” if Trump hobbles Nasa’s climate research.

“He has pushed back the shallow narrative that you can’t grow the economy while cutting carbon pollution,” said Benjamin Houlton, director of the University of California Davis John Muir Institute of the Environment. “He’s shown that’s false. He’s set a model that can transform other countries.”

Brown has been assailed on the right by Republicans, who have claimed his targets are unfeasible and will raise the cost of electricity in the state, as well as some on the left who complain that he has been too willing to appease around oil and gas companies and has failed to phase out drilling in California.

Last week, Brown’s move to stop the Trump administration attempts to allow drilling off California’s coast was dismissed as “pure theater” by May Boeve, executive director of climate campaign group 350.org. “The governor says ‘not here, not now’, but he’s permitted more than 20,000 new oil and gas wells up and down California during his tenure,” she added.

Further concerns have been raised over the stubbornly poor air quality in parts of the state, a problem that’s a particular burden for communities of colour that live near highways and industrial facilities such as oil refineries.

“The critical question now is whether the transition to 100% renewable energy will be a just transition that benefits everyone, especially workers and communities most impacted by pollution and climate change,” said Miya Yoshitani, executive director of the Asian Pacific Environmental Network, which is based in the Bay Area.

Brown himself has echoed the desire for a quickening of pace given the threats California faces – a recent report found the state is set for a major increase in temperatures and sea level, thousands of extra deaths and the loss of two-thirds of its beaches by the end of this century.

This urgency will be spelled out in a major climate summit held in San Francisco this week, as mayors, business leaders and activists from around the world assemble in what will be the largest, and last, gathering on the issue hosted by Brown before he retires to the family ranch.

“Transformation is not for wimps,” he said. “Yes it’s difficult but we are moving at a pace greater than any country. Is it too slow? Yes, but we’re doing everything we can.

“My legacy is restoring my great-grandfather’s ranch. In 10 years there will be a whole new grid, a whole new political class. The whole idea of a legacy is a canard.”